Process and product



United States Patent 3,481,700 PROCESS AND PRODUCT Caslmer C. Legal, Jr., Baltimore, Md., and Harry S. Brackney, Lakeland, Fla., assignors to W. R. Grace & Co., New York, N.Y., a corporation of Connecticut No Drawing. Filed Sept. 1, 1966, Ser. No. 576,535

Int. Cl. C01b 25/22 US. Cl. 23165 3 Claims ABSTRACT OF THE DISCLOSURE A method for preparing a stabilized wet process phosphoric acid having a high P 0 content wherein phosphoric acid is initially concentrated to an intermediate level of dehydration, held for a period to permit precipitation of solid impurities, filtered, and further concentrated.

This invention relates to the treatment of wet process phosphoric acid and is particularly applicable to the stabilization of wet process phosphoric acid.

It is to be understood that throughout this application the phosphorus content is given as the percent P 0 equivalent even though the phosphorus is actually present in the acid solution in different forms.

In the past, it has been common practice to concentrate raw 30% wet process phosphoric acid to the commonly used merchant grade phosphoric acid which has a phosphorus content of approximately 54%. One of the current methods of concentrating wet process phosphoric acid from 30% to 54% is a continuous two-step evaporation procedure in which the 30% phosphoric acid is fed into a first evaporator in which it is concentrated to approximately 4044% P 0 From the first evaporator the phosphoric acid flows directly to a second evaporator which concentrates the phosphoric acid to approximately 54%. The fiow through the evaporators is a continuous flow without any interruption. The only reason for carrying the process out in two stages rather than in one step in a single evaporator is that the present commercial evaporators operate more efficiently at the lower concentration levels and the efficiency of two units in series is greater than if the units are operated separately to concentrate the phosphoric acid to the 54% level.

While 54% phosphoric acid has been the standard merchant grade the actual concentration of phosphoric acid sold as merchant acid has varied from about 52% to 56% P 0 For particular uses other grades of phosphoric acid have been sold. These grades have been below the 52% acid and above the 56% acid and have ranged all the way from the raw acid which is actually from about 27 to 33% P 0 to about 70%.

Each grade of phosphoric acid made from wet process acid presents storage and handling problems because of the precipitates that form in them. These precipitates form scale on equipment and settle out in substantial volume on standing. In a series of commercial shipments of merchant grade acid, about 100 railroad cars, this sediment was found to range from 1 to by weight with an avergae sediment of 4.1%. The sediment problem is less in phosphoric acids having concentrations above 60%. 'In the very popular merchants grade phosphoric acid the sediment problem is, however, very acute.

A large amount of money and extensive research has led to the development of many methods that have been more or less beneficial in reducing precipitation in phosphoric acid containing wet process phosphoric acid. However, the problem has remained a costly one and the various means used to reduce the formation of the precipitate have continued to be costly and not particularly satisfactory. In fact the attempts to solve the precipitaion 3,481,700 Patented Dec. 2, 1969 problem have been so unsatisfactory that a lot of wet process phosphoric acid is still being handled without being treated to reduce precipitation.

An object of this invention is to eliminate or significantly reduce post precipitation in wet process phosphoric acid solutions.

Another object of this invention is to provide an inexpensive process for stabilizing wet process phosphoric acid against post precipitation.

Still another object of this invention is to provide a means that will produce phosphoric acid solutions that will remain stable against :post precipitation for long periods of time.

A still further object of this invention is to provide a simple and inexpensive process for removing impurities from wet process phosphoric acid solutions.

In summary, in a preferred embodiment of one aspect of the invention, the concentration of a wet process phosphoric acid from the raw 27 to 33% concentration to the merchants grade concentration of about 52 to 56% is interrupted at the 38 to 46% level. The precipitates forming at the 38-to 46% level are removed and then concentraiton is continued to the 52 to 56% level.

In other aspects of the preferred embodiment the phosphoric acid that is being concentrated is adjusted to a sulfuric acid content of about 10% of the P 0 cooled at the interrupted concentration, and the acid is subjected to a holding state during which it is agitated for up to about 12 hours, and then held in a quiescent state for up to about 24 hours. Filtration can be substituted in certain instances for part or all of the holding st-a-te.

Further aspects of the present invention will become apparent hereinafter and the specification concludes with claims particularly pointing out and distinctly claiming the subject matter which we regard as our invention. The invention, however, as to organization and method of operation, together with other objects and advantages thereof may best be understood by reference to the following description.

It has been discovered that, in certain instances, when a plurality of samples of wet process phosphoric acid are taken at various concentrations and placed in quiescent storage for a protracted period of time the amount of precipitation formed in each sample is about the same.

It has also been discovered that when the precipitate in the various samples is analyzed the analysis of the precipitate at one concentration has a different analysis than the precipitate at another level of concentration. Table 1 shows the original group of analysis that lead to this discovery. It was then discovered that if the concentration was stopped at intermediate levels, agitated, and the precipitate was removed and then the concentration was carried to higher levels, post precipitation was substantially eliminated.

When raw wet process phosphoric acid which usually has a P 0 concentration of about 27-33% is concentrated to the merchant grade phosphoric acid which is about 52-56% P 0 a good intermediate range for interrupt-ing the concentration to remove the precipitate that will most effectively reduce post precipitation has been found to be about the 38-46% level. It appears that a more optimal intermediate range is 40-44%. If the liquid phase is to be separated from the solid phase by filtration it is generally best to keep the maximum concentration level of the intermediate-interrupted stage of the process below about 40% or 42%. 40-42% phosphoric acid is about the most concentrated grade that lends itself to a reasonable filtering time because of the acids high viscosity when it is more concentrated. Even when settling is used as the method of separating the solid and the liquid phases 44% acid is about the highest concentration with sufficiently low viscosity to permit good settling.

If the intermediate concentration acid is vigorously agitated the first formed small, precipitate particles that form in the acid will be maintained in suspension. These particles serve as nuclei for further crystallization and enhance the rapidity and completeness of the precipitation. The nuclei will, of course, not only increase in size, but accelerate further precipitation of the complex. The larger particles are more easily separated from the liquid phase.

At this time, the most satisfactory commercial means of separating the solid phase of the acid from the liquid phase of the acid is to hold the acid in a substantially quiescent stage. After the particles settle the supernatant liquid phase can be easily recovered by such means as opening an outlet above the precipitate and letting the supernatant run off by gravity or pumping.

After agitation the most rapid settling will occur in the first hours of quiescent holding. Substantial settling will continue for 12 hours and even for 24 hours in many instances. The improvement in the quality of the acid provided by holding the acid in a quiescent state more than 24 hours, generally, does not justify the additional holding time. The mere holding of the intermediate concentration acid is beneficial even without agitation but mostly it is believed for the precipitated gypsum. However, most of the materials that are going to precipitate and settle out readily without agitation which is believed to be mostly gypsum, have usually done so within about 12 hours.

The sludge which forms in the quiescent settling tanks or container may be removed from time to time or it may be removed continuously as it forms. This sludge contains a significant amount of phosphorus and this phosphorus is usually recovered in various ways such as by washing the sludge on a filter to recover most of the phosphorus which it contains. The residue is usually discarded and the wash further refined to a desired product.

If the sulfuric acid level of the wet process phosphoric acid solution is maintained at ten percent of the P it has been found that the precipitate that forms in the intermediate 38 to 46% P 0 acid concentration increases in volume and develops denser and more desirable crystalline character. It is believed that this is because most of the calcium has been converted to gypsum. By maintaining the sulfuric acid level at of the P 0 in the final 54% acid the quantity of precipitation is decreased from that in which the sulfuric acid level is less than 10% of the P 0 Thus, the sulfuric acid acts to increase F precipitation to aid in removal of precipitates in the 38 to 46% acid while decreasing precipitation and aiding in preventing post precipitation in the 54% merchants acid. The sulfuric acid content can usually be conveniently adjusted before the acid is fed into the first evaporator or at the intermediate concentration. In many instances an actual residual sulfuric acid content, or sulfate equivalent, of 3% in the 54% merchants acid will give good results. For best results the sulfuric acid content should be adjusted immediately before or after the acid leaves the evaporator while the acid is still hot.

For best results, it is desirable to adjust the temperature of the phosphoric acid solution from about to 50 C. and more optimally to C. before it is subjected to treatment at the intermediate concentration. Precipitation is more rapid and complete at these lower temperatures than at the usual 100+ C. concentrating temperatures.

It is the combination of providing the correct sulfuric acid content, temperatures, agitation and means for removing the precipitate at the intermediate concentration that provides the optimal process and yields a phosphoric acid product that has the least post precipitation. These various elements employed individually at an intermediate concentration each provides substantial beneficiating of the final product against post precipitation, but it is their combination that provides the best results. It must be understood that many of the time periods vary with the quantities of acid involved and means of treatment employed. The time periods given are generally applicable periods, even for most contemplated plant operations.

In certain instances it may be desirable to provide a storage or holding period after the acid is concentrated to its final concentration before transferring it to railroad tank cars or other shipping containers. This is particularly true if the intermediate concentration treatment had to be shortened or all of the elements of the process could not be employed. This storage period allows any materials in the acid that precipitate rapidly at the terminal concentration to precipitate and settle out. Depending on the depth of the storage tanks and the particular acid involved terminal storage period of from 1 hour to 1 week maybe desirable.

While the preceding discussion relates predominately to a batch type operation the invention is equally useful in a continuous installation. In such an arrangement the intermediate concentration treatment could be carried out in-a continuous flow system in which the acid would flow into a holding zone through which its rate of passage would be slOW. Agitation could be supplied for a portion of the holding zone. The other portion of the holding zone could be a quiet zone having a sluggish, slow moving flow rate. It will be understood that quiet holding includes such a state as a sluggish movement. Of course, if the acid was not at a proper temperature on entering the holding zone, temperature adjusting means could be added in the flow system.

In a continuous system the acid could flow directly from the intermediate or holding zone to the evaporator where it would be concentrated to the desired strength or grade. Then, if the acid could be fed from the evaporator to another zone of relatively quiet flow, this would allow settling of the accumulated solids. The acid could then flow directly from this last zone to long term storage facilities or to shipping containers.

The invention is further illustrated by the following examples which are illustrative only and do not limit the scope of the invention.

EXAMPLE 1 Raw wet process phosphoric acid having a P 0 concentration of approximately 30% was fed into a standard two-stage evaporator installation and concentrated in the first evaporator to about 42% P 0 A 10 liter rubber bucket of this acid was then adjusted to a 6% sulfuric acid content on the basis of 54% acid. This was done by adding sulfuric acid (93%) as needed. About /2 of the acid was removed from the bucket for the procedures of Examples 5 and 6. The remaining 42% acid was then agitated for about 12 hours using a paddle agitator. The agitation was such that all of the first formed precipitate was kept in complete suspension. The acid was cooled to about 30 to 35 C. at the beginning of the agitation period and this temperature was maintained until the acid was ready for further concentration.

After agitation the acid was allowed to stand in a quiescent state for about 24 hours and the precipitate was allowed to settle out. Then about 1 liter of the supernatant was decanted and concentrated to 54% acid in a rotary film evaporator. 250 cc. of the 54% acid was then placed in a 250 cc. glass cylinder and held in a quiescent state for 3 days at room temperature, about 25 C. The post precipitation that settled out was negligible less than /2% EXAMPLE 2 The procedure of Example 1 was carried out with the exception that the acid from the first evaporator had a P 0 concentration of about 39%.

5 The post precipitation that settled out was about l2%.

EXAMPLE 3 The procedures of Example 1 were carried out with the exception that the acid from the first evaporator had a P concentration of about 40%.

The post precipitation thiat settled out was about /z1%.

EXAMPLE 4 The procedures of Example 1 were carried out with the exception that the acid from the first evaporator had a P 0 concentration of about 44%.

The post precipitation that settled out was less than /2%.

EXAMPLE 5 The procedure of Example 1 was carried out with the exception that the acid was not agitated and Was stored for only 12 hours in a glass container. The 42% acid had a clear amber appearance after the 12 hour quiescent period.

The post precipitation that settled out was -16%, which was very bad.

EXAMPLE 6 The procedure of Example 1 was carried out with the exception that the acid was concentrated directly to 54% acid after its removal from the bucket. The 42% acid had a cloudy, dirty appeaarnce when it was taken from the bucket.

The post precipitation that settled out was 18-24%.

EXAMPLE 7 The procedure of Example 1 was carried out execpt the agitation period was only 1 hour, immediately after which the 42% acid was filtered through Whatman No. l filter paper and then concentrated to 54% acid.

The post precipitation that settled out was observed to be bad.

EXAMPLE 8 The procedure of Example 7 was carried out except the agitation period lasted 4 hours.

The post precipitation that settled out was observed to be fair.

EXAMPLE 9 The procedure of Example 7 was carried out except the agitation period lasted 7 hours.

The post precipitation that settled out was observed to be good.

EXAMPLE 10 The procedure of Example 7 was carried out except the agitation period lasted 12 hours.

The post precipitation that would settle out was observed to be very good, not very much, but not quite as good as in Example 1.

6 Table I ANALYSES OF ACIDS ANALYSES OF SOLIDS Weight, Grams Units: Percent of solids by w 'ght:

We claim:

1. A process for concentrating wet process phosphoric acid which comprises:

(a) dehydrating a wet process phosphoric acid containing about 27-33% by weight P 0 to a level of about 38-46% by weight P 0 (b) cooling said dehydrated acid to a temperature of about 20-50 C.;

(c) subjecting said cooled acid to conditions of agitation for a period of from about 4-12 hours;

(d) holding said acid in a quiescent state for a period of about 12-24 hours to permit precipitation of solid impurities therefrom;

(e) separating said acid from said precipitated solid impurities; and

(f) further dehydrating said acid to a level of from about 52-56% by weight P 0 2. The process of claim 1 wherein a sulfuric acid concentration of about 10% by Weight of the final P 0 concentration is maintained.

3. The process of claim 1 wherein said separation step is conducted by filtration.

References Cited UNITED STATES PATENTS HERBERT T. CARTER, Primary Examiner 

